It is no exaggeration to say that miniaturization of a semiconductor integrated circuit pattern has been attained by the progress of photolithography and peripheral technologies. The photolithography, as known well, is supported by roughly two techniques: one is a technique regarding the wavelength of the applied light and the number of openings of a reduced projection light exposure apparatus called a stepper, and the other is a technique regarding resist properties, mainly the resolution of a photoresist composition onto which a mask pattern is to be transferred by the stepper. These two techniques have worked with each other, like two wheels of a car, to improve the accuracy of photolithographic pattern-processing of a semiconductor integrated circuit.
The wavelength of a light source-used in a stepper has been made shorter and shorter in order to satisfy the requirement for higher resolution of a circuit pattern. To obtain a resist resolution of about 0.5 micrometers, generally use is made of a “g” line of a mercury lamp having a main spectrum at 436 nanometers and to obtain a resist resolution of about 0.5 to 0.30 micrometers, use is made of an “i” line of a mercury lamp having a main spectrum at 365 nanometers. Furthermore, to obtain a resist resolution of about 0.30 to 0.15 micrometers, KrF excimer laser of 248 nanometers is used. To obtain a resist resolution of about 0.15 micrometers or less, ArF excimer laser light of 193 nanometers is used. For further reduction, use of F2 excimer laser light of 157 nanometers and Ar2 excimer laser light of 126 nanometers, has been studied.
On the other hand, in view of a photoresist composition, various attempts have been made including the combination of a photoresist composition with an organic or inorganic reflection protecting film, and contrivance of illumination systems. By virtue of these attempts, lithography using KrF excimer laser light has been still in use and therefore a KrF photoresist prolongs its life at the present time. More specifically, a KrF photoresist composition has been developed with a wavelength of λ/2 or less, about 110 nanometers, in view. Also in the lithography using ArF excimer laser light, it has been desired to develop an ArF photoresist composition suitable for a large-scale production of a future design for about not more than 90 nanometer node. Furthermore, the lithography using F2excimer laser light has received attention because it can realize future microfabrication of about 65 nanometers or less. Therefore, the development of a photoresist composition has been proceeding for sufficiently attaining the lithographic microfabrication using F2 excimer laser light.
As is well known, in the lithography, short-wavelength light is applied to a photoresist layer coated on a laminated semiconductor substrate via a mask having a negative pattern or a positive pattern of a desired semiconductor integrated circuit (light exposure). The photoresist layer contains a photosensitive polymer as a main component, which reacts with irradiated light to become insoluble (negative) or soluble (positive) in alkali. After pattern light exposure, the resist layer is subjected to a post exposure baking step to secure the reaction of the resist layer caused by light exposure. Subsequently, the resist layer is developed to remove a soluble portion. As a result, a photoresist pattern layer which accurately reflects the desired circuit pattern is formed on the laminated semiconductor substrate. Thereafter, in some cases, the patterned photoresist layer is subjected to a post baking step to sufficiently cure it in order to give resistance to the following etching step. In the etching step, using the patterned photoresist layer as a mask, the surface layer or the upper layer of the laminated semiconductor substrate is dry-etched along the pattern.
Therefore, the most important property required for a photoresist composition is to provide good resolution. To attain this, patterning light (applied light) must reach not only the surface portion of a resist layer but also the bottom portion near the substrate, thereby allowing light to sufficiently reach the bottom portion of the irradiated resist layer. In other words, the resist layer must have “transparency to irradiation light”.
As is well known, to satisfy the requirements for the lithographic microfabrication as mentioned above, chemical amplification-type resist compositions have been used primarily in recent years. The chemical amplification-type resist composition contains at least a polymer compound having a group highly reactive to acid, an acid generating agent generating acid upon light exposure, and a solvent dissolving them. When the coating film of the resist composition is exposed to patterning light, acid is released from the acid generating agent present in the exposed portion and reacts with the base polymer (polymer compound mentioned above) present in the light exposure portion. In a positive resist composition, the base polymer becomes soluble in alkali since a protecting group (also called as acid-dissociable dissolution suppressing group) is removed from the base polymer by the catalytic action of the acid. Therefore, when the resist layer having a latent image is partially developed by using an alkaline developing solution, a positive-type resist pattern to the mask pattern can be obtained. On the other hand, in a negative resist composition which uses a polymer soluble in a developing solution, the light exposure portion becomes insoluble in the developing solution by the catalytic action of the acid. To the chemical amplification type resist composition, another polymer component serving as a dissolution inhibitor agent may be optionally added. The dissolution inhibitor agent is a polymer having a relatively lower molecular weight of 5,000 or 3,000 or less. The dissolution inhibitor agent, from which a protecting group is removed by an acid generated from an acid generating agent, increasing its solubility in an alkaline developing solution. In other words, the dissolution inhibitor agent suppresses the portion of the resist film remaining as an insoluble portion (non light-exposure portion) from being dissolved in the alkaline developing solution and simultaneously accelerates dissolution of a soluble portion (light exposure portion) by the alkaline developing solution.
To improve the transparency of a chemical amplification type resist composition to light applied thereto, a base polymer and an optionally added polymer serving as a dissolution inhibitor agent must be composed of a polymer compound exhibiting high transparency of the applied light.
The resin components such as the base polymer and dissolution inhibitor agent of a resist composition to be used in the lithography by F2 excimer laser light, which is a light source of the next generation stepper, must have high transparency to the main spectrum (157 nanometers) of the F2 excimer laser light. A desired transparency of the resist film to light having a wavelength of 157 nanometers is disclosed for example in M. K. Crawford et al., “New material for 157 nanometers Photoresists: Characterization and Properties”, Proceedings of SPIE, Vol. 3999 (2000), pp357 to 364. This document reveals that the absorption coefficient (optical coefficient) of a resist film (normalized in thickness) must be equal to or less than 3.0 (μm−1) to obtain a sufficient pattern transfer resolution.
In contrast, a conventional resist material absorbs light having a wavelength of 157 nanometers. In other words, the transparency of the conventional resist material to the applied light having a wavelength of 157 nanometers is low. This means that it is impossible to obtain the next generation resist composition based on the conventional resist materials.
As described above, in the technical field of providing photoresist compositions, it has been investigated to develop a polymer compound having high transparency to light having a wavelength of 157 nanometers. Up to the present, fluorine (F) and silicon (Si) are introduced to a polymer compound to attain the transparency to applied light having the main spectrum at a wavelength of 157 nanometers. Based on this, a novel polymer having not only the transparency but also good resist performances, such as alkaline solubility determining the developing characteristics after light exposure, pattern transfer resolution, and etching resistance, has been developed. However, as to how many percentages of fluorine and silicon-should be introduced, which part of a polymer molecule they should be introduced, and which components of the polymer they should be introduced have not yet been clarified in order to improve the transparency to applied light to a desired level or more while maintaining other characteristics such as dry-etching resistance.
As base polymers for resist compositions having transparency to an ArF excimer laser light, the following polymers are conventionally known: a copolymer of an acrylic ester monomer having an alicyclic group structure and a δ-lactone structure linked to and an androsteron derivative, for example, disclosed in Japanese Patent Application Laid-Open No. 2001-174993; a polymer of a polycyclic unsaturated hydrocarbon derivative which is obtained by subjecting an acrylic acid having an electron-withdrawing substituent at the a position and cyclopentadien or dicyclopentadien to the Diels-Alder reaction to obtain a polycyclic unsaturated carboxylic acid, followed by substituting a hydrogen atom of a hydroxyl group of the unsaturated carboxylic acid by an acid dissociable dissolution suppressing group, for example, disclosed in Japanese Patent Application Laid-Open No. 2001-328964; a copolymer having a monomeric unit derived from an alicyclic hydrocarbon having a polymeric carbon-carbon double bond in the alicyclic ring and a monomeric unit derived from (meta) acrylonitirile, for example, disclosed in Japanese Patent Application Laid-Open No. 2002-196495; a polymer having a monomer having a lactone structure combined to a vinyl group as a monomeric unit, for example, disclosed in Japanese Patent Application Laid-Open No. 2002-278069; and a copolymer of acrylic ester monomer having a fluorine atom at the a position and a vinyl ether derivative, for example, disclosed in Japanese Patent Application Laid-Open No. 2002-293840. In the copolymer disclosed in Japanese Patent Application Laid-Open No. 2001-174993, a fluorine atom is not introduced to improve the transparency. In the polymer disclosed in Japanese Patent Application Laid-Open No. 2001-328964, fluorine is introduced partly in a monomeric unit or in a side chain ring; however, the relationship between the fluorine introduction and the transparency is not clear. In the copolymer disclosed in Japanese Patent Application Laid-Open No. 2002-196495, fluorine is introduced in a part of a terminal-end protecting group of a side chain ring; however, the relationship between the fluorine introduction and the transparency is not clear. In the polymer disclosed in Japanese Patent Application Laid-Open No. 2002-278069, fluorine is not introduced to improve the transparency. In the copolymer disclosed in Japanese Patent Application Laid-Open No. 2002-293840, a copolymer in which part or the about half of hydrogen atoms on a side chain ring are substituted by fluorine atoms is exemplified; however, the relationship between the substitution and the transparency is not clear.
As described above, various polymer compounds have been proposed up to present to improve the transparency to the applied light having a wavelength of 200 nanometers or less. However, it has not yet been elucidated as to which formula of a polymer fluorine should be introduced, which part of the polymer molecule fluorine should be introduced, and how many percentages of fluorine should be introduced have not yet been clarified in order to improve the transparency to light to a desired level or more while maintaining other characteristics such as dry-etching resistance.